The Magazine for Esri Software Users



Measuring in ArcSeconds
Some USGS DEM data is stored in a format that utilizes three, five, or 30 arcseconds of longitude and latitude to register cell values. The geographic reference system treats the globe as if it were a sphere divided into 360 equal parts called degrees. Each degree is subdivided into 60 minutes. Each minute is composed of 60 seconds. Said differently, an arcsecond represents the distance of latitude or longitude traversed on the earth's surface while traveling one second (1/3600th of a degree).
At the equator, an arcsecond of longitude approximately equals an arcsecond of latitude, which is 1/60th of a nautical mile (or 101.27 feet or 30.87 meters). Arcseconds of latitude remain nearly constant, while arcseconds of longitude decrease in a trigonometric cosinebased fashion as one moves toward the earth's poles. At 49 degrees north latitude, along the northern boundary of the Concrete sheet, an arcsecond of longitude equals 30.87 meters * 0.6561 (cos 49°) or 20.250 meters. Ideally, a three arcsecond grid cell on the north edge of the Concrete sheet measures 60.75 meters along its north side and 92.60 meters along its east and west sides.
 